The inner casing of a cold insulating cabinet for a refrigerator, freezer, or ice-making machine has been conventionally manufactured using an impact resistant polystyrene (HIPS) resin or ABS (acrylonitrile-butadiene rubber-styrene) resin and, as an insulation material, a polyurethane foam produced using trichloromonofluoromethane (CFC-11) as the blowing agent.
ABS resin in particular is an optimal inner casing material for said insulating cabinet not only from the standpoints of mechanical properties and processability but also in terms of adhesion to polyurethane foam and stress characteristics on contact with CFC-11 or exposure to the heating-cooling cycle of urethane foam production. However, in view of the recent rigorous control over the production and use of ozonosphere-destroying substances including CFC-11, new blowing agents have been explored. Among the substitutes for CFC-11 so far proposed are 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123) and 1,1-dichloro-1-fluoroethane (HCFC-141b) but since these substitutes are more polar than CFC-11, the ABS resin heretofore available is ready to undergo cracking upon contact with these substitutes.
Thus far, a technique comprising increasing the proportion of the rubber or acrylonitrile component of ABS resin has been proposed. However, increasing the rubber component (Japanese Unexamined Patent Publication No. 132712/1992) sacrifices the rigidity of resin, while increasing the proportion of acrylonitrile (Japanese Unexamined Patent Publication No. 170409/1992, Japanese Unexamined Patent Publication No. 17540/1993, Japanese Unexamined Patent Publication No. 17541/1993) detracts from the excellent mechanical properties and thermal stability of ABS resin. An ABS resin containing an ester of methacrylic acid as a comonomer unit has also been proposed (Japanese Unexamined Patent Publication No. 126756/1992) but this resin is not only inadequate in chlorofluorocarbon resistance but, just as it is the case with the above ABS resin enriched for acrylonitrile, has the disadvantage that the inherently excellent mechanical properties of ABS resin are sacrificed.
Technologies in which ABS resin is formulated with other polymers have also been proposed. For example, incorporation of thermoplastic polyester elastomers (Japanese Unexamined Patent Publication No. 132762/1992, Japanese Unexamined Patent Publication No. 170451/1992), addition of a higher fatty acid triglyceride to acrylonitrile-enriched ABS resin (Japanese Unexamined Patent Publication No. 154858/1992), the use of acrylonitrile-enriched acrylic rubber-modified graft and butadiene rubber-modified graft in combination (Japanese Unexamined Patent Publication No. 170460/1992), the use of ethylene-propylene rubber-modified graft and butadiene rubber-modified graft in combination (Japanese Unexamined Patent Publication No. 170461/1992), the use of ethylene-propylene rubber-modified graft and acrylic rubber-modified graft in combination (Japanese Unexamined Patent Publication No. 170462/1992), and addition of a polyester (Japanese Unexamined Patent Publication No. 17658/1993) can be mentioned by way of example. Although these technologies certainly contribute to chlorofluorocarbon resistance, none of them are fully satisfactory in the effect achieved.
In addition, the use of an agglomerate for the rubber or graft component of ABS resin has also been proposed. Thus, the technology employing an agglomerated rubber as the rubber component (Japanese Examined Patent Publication No. 30034/1977), the technology which comprises grafting a small proportion of a compound to the rubber component (a low grafting degree) and causing the resulting graft polymer to undergo agglomeration and enlargement (Japanese Examined Patent Publication No. 27378/1990), and the technology which comprises grafting a small proportion of a compound to the rubber component, causing the resulting graft polymer to undergo agglomeration and enlargement to form clusters, and finally grafting a larger amount of the compound (Japanese Unexamined Patent Publication No. 25227/1993) can be mentioned by way of example. These technologies are also contributory to enhanced chlorofluorocarbon resistance but are never fully satisfactory.
There has, thus, been a long-standing need for a new material having improved chlorofluorocarbon resistance (i.e. not easily undergoing cracking on contact with said substitutes) without being compromised in the characteristically excellent mechanical properties, thermal stability and processability of ABS resin.